1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus capable of effectively cooling down a recording medium after a fixing process with heat and pressure.
2. Discussion of the Background Art
In general, an electrophotographic method is widly used in an image forming apparatus such as a copying machine, a facsimile machine, a printer, a multi-function machine, and the like. The electrophotographic method employs a series of processes such as charging, exposing, developing, transferring, fixing, and so on, to finally produce an image on a recording medium (e.g., a recording sheet). The discussion here focuses on the fixing process that follows the transferring process. In the transferring process, a recording medium receives a toner image from a photosensitive member generally by an electrostatic force. The toner image transferred onto the recording medium is unfixed but is held on the surface of the recording medium by the electrostatic force. Such a recording medium carrying an unfixed toner image thereon is then subjected to the fixing process. The fixing process typically apply heat and pressure to melt the toner and to press the melted toner onto the recording medium.
As such, the recording medium usually has a relatively high temperature after the fixing process. This phenomenon becomes evident, particularly when image are reproduced at a relatively high speed. Therefore, a high-speed image forming apparatus has been facing a problem called a blocking. This problem occurs on recording sheets having a relatively high temperature after the fixing process. More specifically, the toner image carried on the recording medium may partly be still melted and therefore fixed to another sheet. That is, the recording sheets are adhered to each other.
Several attempts to address this problem may be referred to Japanese Utility Patent No. 2542935 and Japanese Unexamined Patent Application Publication No. JP2003-241623, for example. These references describe a cooling system which uses a heat pipe for drawing heat from the heated recording medium, and a radiating fin connected to the heat pipe and radiating heat transmitted from the heat pipe. The radiating fin is encased in a duct which has an air inlet for taking in a fresh air and an air outlet for ejecting a heated air.
In this cooling system using the heat pipe and the radiating fin, in particular, a forced air cooling to cool off the radiating fin has the largest terminal resistance among other components. Accordingly, efficiently cooling the radiating fin is needed to improve a total cooling efficiency of the cooling system. Although using a cooling fan of a higher rating may be an instant solution, it may lead to an environmental problem such as an increase of a manufacturing cost and a noise.
In a conventional background image forming apparatus, a radiating fin having a plurality of disc-like-shaped fins is encased in a cooling duct and is connected to a heat pipe which rotates together with the radiating fin when drawing heat from a recording sheet. The heat of the recording sheet is transmitted through the heat pipe to the plurality of fins of the radiating fin. In the cooling duct, air is blown to the plurality of fins of the radiating fin so as to cool down the fins.
FIG. 1 illustrates a typical air flow in a cooling duct encasing a radiating fin 1. As illustrated in FIG. 1, the air flow is divided into two air streams when impinging on the radiating fin 1 in the cooling duct. One air stream clearly appears to enter into gaps between fins of the radiating fin 1. In this part of the radiating fin 1, the air stream flows in a direction substantially same as a rotation direction of the radiating fin 1. However, the other air stream appears to flow away from the radiating fin 1. In this part of the radiating fin 1, the air stream flows in a direction substantially opposite to a rotation direction of the radiating fin 1. That is, it is experimentally understood that the forced air cooling does not use almost a half of the air flow.
In addition, FIG. 2 illustrates a profile of an air flow speed generated by a cooling fan 2. As illustrated in FIG. 2, the air flow speed is not flat, that is, two peaks on edges sandwiches a flat hollow. Specifically, the air flow speed has a peak on a circumferential region of the cooling fan, and stays a relatively low level in an inner radius region. When such an air flow having an uneven speed impinges on the radiating fin, a middle part of the radiating fin may not efficiently be cooled down.